llvm dev - Jun 2010 - [LLVMdev] XMM in X86 Backend

Hi all,

I am observing an excessive use of xmm registers in the output assembly
produced by x86 backend.  Basically, for a code like this

double test(double a, double b) {
 double c;

 c = 1.0 + sin (a + b*b);

 return c;
}


llc produced somthing like....

movsd   16(%ebp), %xmm0
       mulsd   %xmm0, %xmm0
       addsd   8(%ebp), %xmm0
       movsd   %xmm0, (%esp)
       .......
       fstpl   -8(%ebp
       movsd   -8(%ebp), %xmm0
       addsd   .LC1, %xmm0
       movsd   %xmm0, -8(%ebp)
       fldl    -8(%ebp)


LLVM Backend is using xmms it involves a lot of register moves. llc has one
option -mcpu=686, where output does not use xmm but it disables some other
instructions also. Is there any fine grain optimization flag like ( -mfpmath
= 387 which is present in gcc) which would just instruction backend to use
fpstack instead of SSE instructions and still not limit the instructions

Thanks

--Kapil

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On Jun 7, 2010, at 10:04 AMPDT, kapil anand wrote:
>
> LLVM Backend is using xmms it involves a lot of register moves. llc  
> has one option -mcpu=686, where output does not use xmm but it  
> disables some other instructions also. Is there any fine grain  
> optimization flag like ( -mfpmath = 387 which is present in gcc)  
> which would just instruction backend to use fpstack instead of SSE  
> instructions and still not limit the instructions
-mattr=-sse

Note that this doesn't change the calling convention; in particular,  
the only supported calling convention for 64-bit x86 uses SSE, so  
you'll get an error if you use the above for a function returning  
double in 64-bit mode.